The invention lies in the boiling water reactor technology field. More specifically, the invention relates to a method for protecting the components of the primary system of a boiling water reactor in particular from stress corrosion. In a boiling water reactor, the coolant which comes into contact with the reactor core is known as primary coolant, and the lines and components which are exposed to the primary coolant are known as the primary system. In addition to the reactor pressure vessel, the primary system of a boiling water reactor includes systems of lines as well as various internal fittings and pumps. The components generally consist of stainless steel, for example of a CrNi steel, or an Ni-base alloy, such as Inconel® 600 (Inco Alloys International, Inc.). Radiolysis of the primary coolant causes, inter alia, the reaction products hydrogen peroxide, oxygen, and hydrogen to form in the boiling water reactor. The oxidizing conditions which result from the excess of oxidizing agents promote corrosion, in particular stress corrosion cracking, of the components. To remedy this, it is known to admix hydrogen with the primary coolant. This bonds oxidizing agents contained in the primary coolant and shifts the electrochemical potential of the component surfaces toward negative values. A drawback of the conventional method is that relatively large quantities of hydrogen are required to ensure sufficient protection against corrosion. The high demand for hydrogen, which entails corresponding costs, is attributable not least to the fact that the electrochemical oxidation of the hydrogen on the component surfaces which are covered with an oxide layer is subject to considerable reaction inhibition, and this has to be compensated for by increased hydrogen concentrations. A further drawback is the outlay on apparatus for metering the gaseous hydrogen.
European patent disclosure EP 0736878 describes a method in which the oxide layer of the component surfaces in the primary System is doped with precious metal, which makes it possible to use smaller quantities of hydrogen. German published patent application DE 100 30 726 A1 describes a method in which the quantities of hydrogen and precious metal are supposed to be reduced by coating the component surfaces with a film which includes a substance with a photocatalytic action. The substances with photocatalytic action that are used—preferably Ti02 and Zr02—are N-type semiconductors which are excited by the Cherenkov radiation which is present in the reactor, shifting the corrosion potential of the component surfaces toward negative values.
Soviet Union patent disclosure SU 653953 describes a system having to do with what is referred to in the document as “boiling nuclear reactors.” There, an alcohol is added into the primary coolant instead of hydrazine. While relatively little information is presented in the document concerning the operational setup of the reactor, certain statements strongly suggest that the boiling reactor of the prior art publication is not a boiling water reactor according to the Western understanding. One such hint is that the publication states that, in its prior art, hydrazine had been introduced in such boiling nuclear reactors during the reactor operation for the purpose of providing corrosion protection. The addition of hydrazine, however, during the operation of a boiling water reactor would be entirely prohibited.
The Soviet document discloses corrosion protection measures by way of the addition of alcohol in the coolant/moderator. The specific concentration disclosed is approximately 10 to 105 μmol/kg (≈0.32 to 3200 ppm for methanol) in order to completely prevent oxygen formation during the radiolysis of the coolant. In order to ensure this, the disclosed alcohol concentration must necessarily be present at those locations at which the radiolysis processes are the strongest, that is, at the fuel rods in the reactor core.
A problem associated with very high alcohol concentration is that a relatively large portion of the alcohol remains unused, i.e., it is not oxidized by radiolysis oxygen or decomposed by the radiolysis, it subsequently passes through a phase change into the vapor phase and then reaches the steam turbine and the condenser downstream of the steam turbine. There, the alcohol is cooled to about 40° C. At this temperature, only a small proportion of the alcohol is dissolved in the liquefied condensate which is fed back into the reactor pressure vessel in the form feedwater. The by far largest proportion is contained in the vapor phase. The latter is not simply let go into the environment but it is transported via an off-gas path within which a catalytic recombination of hydrogen and oxygen to water is effected. An alcohol component in the vapor phase could, on the one hand, disturb the recombination. On the other hand, additional functional elements and processing steps would have to be provided in order to hold back the alcohol or to convert the same into a non-damaging form.
High alcohol contents, furthermore, lead to radiolysis in the reactor due to the high radiation density, which results in products such as CO2, formaldehyde, and formic acid. These products, of course, are undesirable in the reactor pressure vessel itself and in the downstream vapor carrying systems such as the condenser. Besides an increase in the conductivity of the primary coolant, they can lead to a decrease in the pH which has a negative effect on the component corrosion. Yet, it is exactly the component corrosion which is to be avoided or reduced with the addition of alcohol.